The incidence of bacterial infections, bacterial contaminations, and the spread of drug-resistant bacteria represent growing worldwide health problems. For example, over 8.8 million new cases of tuberculosis are estimated to occur worldwide per year. Recent news has also highlighted the emergence of resistant strains of tuberculosis, which pose a danger to travelers using mass transit, such as commercial airplanes. Rapid detection of the presence or absence of bacteria, of the quantity or bacterial load of bacteria, and of resistant strains of bacteria, such as tuberculosis, is a top priority of health organizations such as the World Health Organization.
Current methods of detecting the presence or absence of bacteria typically include culturing a sample suspected of having bacteria followed by isolation and use of various biochemical tests. These tests may take between 2-21 days to complete depending on how long it takes to isolate and grow the bacteria. Accordingly, while such biochemical testing is relatively inexpensive, it is time consuming to grow and subculture bacteria in a sample to reach the minimal concentration of bacteria needed for testing.
There are PCR-based methods for rapidly detecting bacteria, but these typically are expensive and require advanced laboratory equipment and techniques and various reagents for use. Furthermore, there typically is frequent contamination of samples that precludes use in more resource-limited settings. Nucleic acid amplification (NAA) systems have been used in industrialized countries for some time, and two systems have been approved by the FDA for MTb detection in sputum. NAA is more sensitive than a smear microscopy test, but less sensitive than culture. NAA, though better than smear microscopy, is expensive and typically requires considerable technical support and quality control. For example, rapid sequencing of bacteria for diagnosis and drug resistance determination (i.e. using PCR) is difficult to make portable and robust for field use. It typically requires significant power and reagents that require special treatment (i.e., cold storage). In addition, even in strictly maintained laboratories there still may be contamination leading to false positive tests. These downfalls make it an unlikely target for a system to be used in high-need (developing) countries.
By way of example, Mycobacterium tuberculosis (herein also referred to as “TB,” “MTb,” or “M. tuberculosis”) diagnostic standards have not changed significantly in the past century. To the extent that new MTb diagnostics have been developed, they typically are not practical for wide-scale use, for example, in third-world countries. One standard for the diagnosis of active pulmonary tuberculosis is sputum smear microscopy for acid-fast bacilli. If a patient's sputum tests positive for MTb (considered “smear-positive”), they have active pulmonary tuberculosis, are considered highly infectious, and are placed on an exhaustive drug regimen for treatment. However, sputum smear microscopy has low sensitivity and typically requires appropriately trained personnel to accomplish. In fact, it is estimated that sputum smear microscopy at best detects 25-60% of people with active pulmonary tuberculosis. The method also has relatively poor limits of detection as it requires the presence of at least 10,000 MTb bacilli/mL.
Serologic tests exist for MTb diagnostics, but they continue to undergo development and tend to be more specific for exposure than active disease. Some commercialized tests use immunodominant antigens to detect immunoglobulin classes (like IgG) in an ELISA or dipstick format. Serological tests are estimated to detect one-third to three-quarters of sputum smear-positive cases of MTb. They detect a significantly smaller portion of smear-negative cases with HIV co-infection. In fact, for people infected with both HIV and MTb, serological tests detect less than one third of patients with the active form of the disease.
Phage systems that detect live mycobacteria in liquid cultures using phages that act as indicators by infecting and replicating in MTb cells have been developed. Phage systems appear to be fast, robust and highly sensitive, but little is known about their reproducibility and performance. Phage systems, though highly promising for their speed, robustness, and high sensitivity, typically require, in use, the presence of skilled professionals and may turn out to be very costly. Furthermore, phage systems appear to be inhibited by a factor in sputum, thus rendering them less useful because sputum is the sample frequently used for diagnosis. Accordingly, the systems may not lend themselves well to widespread use in developing countries.
Radiometric and fluorescent liquid culture systems, often used in level III laboratories, are highly sensitive, but also may require support of a full microbiology laboratory, typically require relatively long times (1-3 weeks) to generate results, and are relatively expensive to purchase. Radiometric liquid culture systems, though robust and sensitive, require radioactive materials, which therefore typically require special facilities and training for their use. The cost of materials also may be very high and the systems not portable.
Some non-standardized culture systems have been developed that employ inexpensive reagents and are more suitable for widespread use, but more studies are needed to determine the accuracy of these systems. At the very least, they have demonstrated performance levels comparable to standard diagnostics methods. These non-commercial liquid culture growth detection methods that employ inexpensive reagents may be more suitable for use in developing countries, but they are not yet standardized and thus have not been readily endorsed by TB diagnostics experts. They may benefit from standardization of reagents, packaging, and product support.
Another common MTb test is the tuberculin or purified protein derivative (PPD) (PPD skin test), which is the skin test developed for the screening of latent MTb. Additional screens for latent MTb now include new in vitro assays that measure IFN-γ produced by T lymphocytes in whole blood after stimulation from PPDs obtained from MTb, M. avium and M. bovis. Single specific antigens have been used to increase specificity as well. The tuberculin or PPD skin test shares many antigens with a common tuberculosis vaccine, Bacillus Calmette-Guerin (“BCG”), and environmental bacteria so people without latent MTb infection frequently test positive. This approach is further complicated by the need for a clinician to interpret the results and for multiple visits to a clinic by the patient to obtain the results. The skin test frequently has unreliable results in many patients including those having received a MTb vaccination or those infected with another type of mycobacteria. HIV/AIDS patients frequently test negative when they are also carriers of MTb. Additional skin tests in development (like the one that measures IFN-γ) are more specific, though not perfect, and increasing the specificity of these tests is often at the cost of sensitivity. These tests are also not useful for detecting active MTb infection, but primarily show exposure or latent MTb infection
Accordingly, rapid point-of-care bacterial detection devices, methods and systems are needed, for example, to screen patients suspected of one or more bacterial infections. Preferably, such devices, methods and systems can be used in the field, for example, for onsite rapid monitoring of the bacterial infections of humans or animals (e.g., in developing countries or any location removed from a laboratory setting).